The present invention is an improved method of producing an aluminum tube covered with a layer of zinc, which is usable in a heat-exchanger or similar product and which has excellent corrosion resistance, through the use of a continuous cold forming machine.
Conventionally, a flat aluminum tube or a round aluminum tube for a heat-exchanger has been coiled with a winder after extrusion through a hot extruder.
With this method, however, a seam is caused for every extruded billet and air or oil penetrates into the seam portion resulting in many defects, frequently referred to as blisters. Hence, this method cannot be used for producing a heat-exchanger wherein a high degree of pressure endurance and corrosion resistance are required, and the weight of the billet providing the material is greater than necessity to obtain a defect-free coil. Namely, a seam exists usually for every coil of 30 to 50 kg and the portion having the seam cannot be reliably used where pressure endurance and corrosion resistance are required. The seamed portion is usually discarded.
A method for improving the corrosion resistance of such an aluminum tube has been proposed. For example, as shown in Japanese Unexamined Patent Publication No. Sho 58-204169, using aluminum material as a metal extrusion material, Zn was flame-sprayed onto the surface of the aluminum material in the vicinity of the extruding outlet of a hot or cold extrusion forming machine to form a Zn layer on the aluminum material.
However, even though an anticorrosive aluminum tube may be produced by this method, it has been impossible to produce a coil comprising a long seamless aluminum tube for the reasons described above.
As a further improvement, another method of producing aluminum tube by using a continuous cold forming machine was recently proposed. For example, as shown in Japanese Unexamined Patent Publication No. Sho 60-1087, this method utilizes prime materials such as aluminum etc. which are supplied into a long and narrow pathway formed from a mandrel groove provided on the circumference of a movable wheel and a fixed seal block engaging with the groove. The prime materials are fed compulsively into the pathway by the contact and friction resistance between the inner face of the groove of the rotating movable wheel and the prime materials to generate an extruding pressure on the prime materials, and extruded aluminum tubes are produced through a die attached to a forward end of the machine.
According to this method, it is possible to produce a seamless coil having a weight 10 to 20 times heavier than a coil produced by the method of producing aluminum tube through a hot extruder as previously described.
In this method, however, since the surface temperature of the aluminum tube extruded from the continuous cold forming machine is as low as 200.degree. to 400.degree. C., the Zn does not diffuse sufficiently into the surface when it is sprayed onto the surface which results in a poor adhesion strength between the aluminum tube and the Zn. Thus, it has been impossible to accomplish the purpose of providing a satisfactory Zn-covered layer on an aluminum tube.
As a result of diligent studies in view of this situation, a seamless aluminum tube has been produced through the use of a continuous cold forming machine and a method of producing a Zn-covered aluminum tube provided with an anticorrosive layer of Zn on the outer surface thereof, which is excellent in adhesion strength, has been developed according to the invention.